1. Field of the Invention
The present invention relates to radiating Ultra Wide Bandwidth (UWB) radio frequency (RF) pulses through an antenna for use in communication systems and sensors such as radars and, more particularly, for use in impulse radar systems.
2. Description of the Prior Art
Impulse radar is used in a variety of radar systems in order to determine the location of aircraft, ground vehicles, people, mines, buried pipes, roadway faults and tunnels. A problem that arises in connection with the use of impulse radar is the reflection of the transmitted pulse from the transmitting antenna itself, due to poor return loss in the antenna. Return loss is defined to be the ratio of the energy reflected relative to the energy incident on a port. The reflected pulse causes an exponentially decaying oscillation as the pulse reflects back and forth between the transmitter and the antenna. This oscillation can be so strong and last so long that it masks the intended target.
Common methods for minimizing this oscillation problem are: (1) add a loss to the transmission line; (2) add a loss to the antenna; and (3) design a transmitter that will terminate the reflected wave. Approaches (1) and (2) have the disadvantage of wasting a portion of the transmitter power. Approach (3) has resulted in inefficient bulky designs.
Wicks and Antonik in a paper entitled "Polarization Diverse Ultra-Wideband Antenna Technology" describe a UWB antenna design having a truncated cone shaped ground plane surrounded symmetrically by four radiating elements. This paper is expressly incorporated by reference into the instant application. FIG. 9 of the paper shows a feed arrangement with an adjustable phase shifter for achieving multiple polarizations. FIG. 4 of the paper shows this antenna does not have a substantially flat impedance as time approaches infinity. This paper is contained in Ultra-Wideband, Short-Pulse Electromagnetics, edited by H. Bertoni et al., Plenum Press, 1993.
An effective solution for approach (2) involves adding a resistively loaded parallel plate section to the end of a Transverse Electric Magnetic (TEM) horn and adding a shunt network having a shorting wire in series with resistors in the resistively loaded parallel plate section, as set forth in pending U.S. patent application Ser. No. 08/160,304 by the instant inventor. Nonetheless, it is desirable to have an antenna that further minimizes losses while achieving higher return loss and more gain.